The present invention relates to a misregistration correcting method and device for correcting deviations of transfer positions of images of respective colors in an image forming apparatus which produces a multicolor image by transferring the images of the respective colors formed by a plurality of image forming sections onto a single recording medium while sequentially superimposing the images one upon another. The invention also relates to the above image forming apparatus.
Conventional electrophotographic multicolor image forming apparatuses produce a multicolor image by sequentially superimposing a plurality of toner images having different colors. For example, in one of such multicolor image forming apparatuses, a toner image of each color is transferred onto a recording sheet every time it is formed on a single photoreceptor, whereby the toner images are superimposed one upon another on the recording sheet.
To form a full-color image in this type of image forming apparatus, it is necessary to use a scanning optical system to perform image exposure of three or four colors and developing units for the respective colors, and to transfer a toner image onto a single recording sheet with a transfer unit every time it is formed. To this end, toner images of the respective colors are sequentially transferred while being superimposed one upon another by forwarding a recording sheet plural times to the location confronting the transfer unit by using a medium carrying and transporting unit that circulates while carrying the recording sheet.
To shorten the time required for multicolor image formation, there has been proposed an image forming apparatus which forms toner images of a plurality of colors by means of image forming sections, each having a scanning optical system for image exposure and a developing unit for each color. This image forming apparatus has, e.g., a transfer belt (medium carrying and transporting unit) for transporting a recording sheet while carrying it, and a plurality of photoreceptors are arranged around the transfer belt. Toner images of a plurality of colors are formed by charging units, scanning optical systems, and developing units that are arranged so as to confront the respective photoreceptors, and sequentially transferred onto a recording sheet.
In this type of image forming apparatus, toner images of different colors formed by a plurality of image forming sections need to be transferred onto a single recording sheet so as to be superimposed one upon another exactly. This is so because toner images with deviated transfer positions causes a color deviation, which greatly deteriorates the quality of a resulting full-color image.
The color deviation may be caused by errors in the locations at which a plurality of image forming sections are installed, errors in the position at which an image is written onto the photoreceptor in each image forming section, and other errors. The types of color deviations include: a transfer image deviation in the moving direction of the medium carrying and transporting unit; a transfer image deviation in the direction perpendicular to the above direction (main scanning direction); and range enlargement and reduction in the main scanning direction.
To eliminate a color deviation, there have been proposed in an image forming apparatus misregistration correcting methods for correcting transfer position deviations of toner images of different colors.
In one misregistration correcting method, misregistration amount detecting pattern images formed by the respective image forming sections are transferred onto the medium carrying and transporting unit. Deviations of the pattern images are measured at one fixed position, based on which deviations of the image writing positions are corrected. In another misregistration correcting method, misregistration amounts are measured at a plurality of positions of the circumferential surface of the medium carrying and transporting unit, and averages for one revolution is calculated as shown in FIG. 5, based on which averages the image writing positions are corrected. In these misregistration correcting methods, the transfer position of an image is corrected to an appropriate position by changing the image writing position by controlling the image light irradiating timing in the image forming section for each color based on the measured or calculated misregistration amount.
However, the aforementioned misregistration correcting methods for an image forming apparatus have the following problems.
The amounts of misregistration produced in transferring toner images formed by a plurality of image forming sections are not uniform over the circumferential surface of the medium carrying and transporting unit. For example, in the case of using the aforementioned transfer belt, the transfer belt has such a circumferential length as to allow a plurality of sheets to be placed thereon. The misregistration amount differs depending upon the location of the transfer belt at which a recording sheet to receive toner images is placed. The misregistration amount exhibits periodicity, i.e., varies with a cycle that corresponds to one revolution of the transfer belt, as shown in FIG. 5.
Thus, even if the image writing positions are corrected by forming misregistration amount detecting pattern images on the medium carrying and transporting unit and measuring actual misregistration amounts, whether misregistration can be corrected properly or not depends on the location at which a recording sheet to receive images is placed, in either of the above two methods, i.e., the method of measuring misregistration amounts at a single fixed position, and the method of measuring misregistration amounts at a plurality of positions of the circumferential surface of the medium carrying and transporting unit and calculating an average of the misregistration amounts as shown in FIG. 5. As a result, the quality of a produced image is impaired due to color deviations, making it impossible to provide high-quality images in a stable manner.